Our recent studies have indicated that there are two types of cardiac myosin, each with a distinctive pattern in regard to stability, response to sulfhydryl reagents, activation energy and adenosinetriphosphatase activity, and the level of thyroid hormone appears to determine which type of myosin is synthesized. We will attempt to characterize the 2 types of cardiac myosin further and to study other factors influencing this molecule. Studies on the rate of synthesis and turnover of the heavy and 2 light chains of cardiac myosin and on hormonal influences on the rate will be continued. Studies on the influence of growth-promoting factors in serum on cell multiplication in rat aorta will also be continued.